Should I start an SIP when the market is depressed or when it is booming?

It is much better to do SIPs in a downtrending market than an uptrending one. Why? Because your average price wil be lower overtime. Most people load up on SIPs in up trending markets and it's a sure shot way to earn mediocre returns. Such investors will have to sit tight atleast for 2 cycles to make money in my opinion. 

There is a quote with the same meaning " Buy the DIP and Sell the RIP" 

And the common mistakes many new investors do is that, pausing the SIPs when marketing is falling, thus they miss the golden opportunity to average the price. 

SIP determines your commitment to your financial health. So whether you invest in any market conditions it is always good.

1. If you invest a lumpsum in market depressed situations, you will get more units at less NAV (Mutual Funds) and viceversa incase of market raising conditions.
2. So dont confused over market conditions.